A substantial percentage of cancer patients is effected by skeletal metastases. As many as 85% of patients with advanced lung, prostate and breast carcinoma develop bony metastates (Garret 1993, Nielsen et al, 1991). They are associated with a decline in health and quality of life, ultimately leading to death, often within a few years.
When tumors or metastases cannot be removed by surgery, the conventional approach is to apply external beam radiotherapy and chemotherapy. Both suffer from a lack of selectivity for tumor cells and tumor tissue. As a consequence, treatment most often cannot be applied at curative levels due to toxicity to healthy tissue.
Bone-seeking β-emitters like 89Sr and 153Sm complexed with ethylene-diaminetetramethylene-phosphonate (EDTMP) have been used as internal radiotherapy agents in the pain palliation of painful bone metastases especially in prostate cancer. The altered skeletal metabolic activity around many bone metastases results in a local increase in bone formation and uptake of calcium, which is used to construct the hydroxyapatite bone mineral. Bone-seeking radionuclides target this bone adjacent to the tumor deposits. Calcium mimetics, such as strontium 89Sr, belong to the alkaline earth group of elements in the periodic table. They can be administered as an intravenous radioactive salt that will be incorporated into the newly formed hydroxyapatite in bone metastases. Other radionuclides, such as 153Sm, require a carrier molecule to achieve selective uptake to the bone, for example, EDTMP. By selectively targeting areas of high metabolic activity in bone, a high therapeutic index is possible.
However, the β-particles are characterized by low-linear energy transfer (LET) typically in the range of 0.2-1.0 keV/μm and a modest relative biological effectiveness (RBE). The use of highly energetic β-particles is restricted by the radiation burden and cell damage to surrounding healthy tissue and especially by the suppression of blood cells in the red bone marrow. Hence, there is an unmet need for more effective bone-targeted treatments that improve quality of life and survival whilst maintaining a favorable safety profile.
The use of α-emitting radionuclides has a major advantage in radiotherapy of cancer. Compared to the low LET values of β-emitters, α-emitters have a mean LET value of 80-100 keV/μm. 223Ra has shown particular promise. For example, Alpharadin® (223RaCl2) has completed a global phase-III clinical trial in patients with castration-resistant prostate cancer (CRPC) and bone metastases. Data shows that Alpharadin prolongs patient overall survival time while offering a well tolerated safety profile (Brady et al, Cancer J., 2013, 19, 71-78). 223Ra, like 89Sr, is a calcium mimic and also an alkaline earth element and can be administered as an intravenous radioactive salt. Due to the high LET-values of α-particles and, consequently, their short path-length in human tissue (<100 μm), a highly cytotoxic radiation-dose can be delivered to targeted cancer cells, while damage to the surrounding healthy tissue is limited.
Quality control is an essential part of pharmaceutical manufacture, to ensure the drugs sent to the market are safe and therapeutically active formulations have a performance which is consistent and predictable. The term quality control refers to the sum of all procedures undertaken on each batch to ensure e.g. the identity, activity and purity.
Radionuclidic purity is defined as the percentage of a contaminating radionuclide relative to the wanted radionuclide e.g. 227Ac relative to 223Ra with respect to activity in Bq. The primary reason for seeking radionuclidic purity in a radiopharmaceutical is to avoid unwanted administration of radiation to the patient. It is therefore extremely important to strictly control the levels of radionuclidic impurities in radiopharmaceuticals. Radionuclidic impurities may originate from several sources. For example, when a parent-daughter radionuclide generator system is used to produce the radionuclide of interest, the parent nuclides are defined as impurities in the product. Actions must be taken during production to ensure that the parent nuclides are separated from the nuclide of interest and, before release of the finished product for human use, it has to be confirmed that the radioactivity of the radionuclidic impurities are below the limit specified for the product.
Production of 223Ra for pharmaceutical use is typically based on a radionuclide generator where the mother nuclide 227Ac (t1/2=21.77 years) is adsorbed on a column material. The daughter radionuclides are 227Th (t1/2=18.68 days) and 223Ra (t1/2=11.43 days). 223Ra is separated by column elution. 227Ac and its daughter nuclide 227Th must be strongly retained under conditions were 223Ra can be eluted. 227Ac and 227Th do not have the same bone seeking properties as 223Ra and are regarded as impurities. Even very low amounts of these nuclides cannot be accepted in the pharmaceutical product. The acceptance criterion for Alpharadin has been set to not more than 0.004% for 227Ac and not more than 0.5% for 227Th relative to 223Ra with respect to activity in Bq. Similar criteria would be expected for other 223Ra products. Prior to formal release of the product to patients, each produced batch of radiopharmaceutical (e.g. Alpharadin) must be tested to show that it meets the acceptance criteria (adequately defined identity, strength, quality and purity). Due to the inherently short half-life of 223Ra, the radiopharmaceutical may be released before completion of all tests (e.g. sterility testing). This naturally has the disadvantage that patients could be exposed to a formulation which does not meet all the quality control criteria.
A quantitative determination of 227Ac is difficult as 227Ac decays almost entirely by emission of a low-energy β-particle (Eβ,max=0.0448 MeV), which is virtually undetectable in the presence of all the energetic α- and β-emitters of the 227Ac chain (see FIG. 1). 227Ac also decays by α-emission in 1.38% of its disintegrations. However, direct α-spectrometric determination of 227Ac is complicated by interferences from the α-emissions of its rapidly growing decay products. Freshly purified 227Ac emits no analytically useful γ-radiation.
Consequently, many radiometric methods determine 227Ac indirectly by measurements of the α- and γ-radiations of its daughters, in particular by high-resolution γ-spectrometry of its daughter 227Th. However, this cannot be determined until 10-12 months after release of the product as analysis must wait until there are sufficiently measurable levels of 227Th. At this time, the potential amount of 227Ac contamination is in equilibrium with its daughter 227Th. Furthermore, the initial amounts of 223Ra and any 227Th in the product would have decayed completely. These disadvantages not only lead to inaccuracy of results and increased costs but, more significantly, mean that the result comes too late for the 223Ra pharmaceutical to be withdrawn from release to patients should it be shown to be contaminated with 227Ac at levels which would be considered to jeopardise the efficacy of the treatment or the safety of the patient.
In view of the above, there remains a need to develop a new, reliable, accurate and cost-effective radiochemical method for early determination of the potential contamination of 227Ac in 223Ra pharmaceuticals, such as Alpharadin (RaCl2). In particular, it would be an advantage to produce a method which is able to give a result in a matter of days rather than months. Ultimately, an analysis method which can be completed prior to release of the product and its administration to patients is attractive. The following criteria set out the desirable features of a new quantification method:                1. 227Ac should selectively be separated from the precursors.        2. Recovery of 227Ac>70% and precision>30%        3. Robustness i.e. the analytical result should remain unaffected by small variations in method parameters.        4. Easy to operate in routine production (in terms of time and cost).        5. Sample activity should be as low as possible due to cost and radiation exposure to the operators, and/or        6. Separation and quantification should be fulfilled before release of the product i.e. within 2 days after production of the 223Ra pharmaceutical (e.g. 223-radium chloride).        
The present inventors have surprisingly found that an analytical method employing a tandem column arrangement comprising two different solid phase extraction resins can fulfil some or all of these requirements. In particular, the two columns enable facile separation and isolation of 227Ac, which can be rapidly quantified.